1. Field of the Invention
The invention relates to a method and arrangement for detecting a watermark in a signal.
2. Description of the Related Art
Watermarking of audio and video material is an important building block in copy protection schemes. A watermark is an additional piece of information which is embedded in the content. Preferably, the embedded watermark is imperceptible, survives A/D and D/A conversion, survives compression techniques, and is robust against various signal- processing algorithms.
A watermark is generally embedded in a signal by modifying samples of the signal in accordance with respective samples of the watermark. The expression xe2x80x9csamplesxe2x80x9d refers to signal values in the domain in which the watermark is embedded. Samples may be, for example, temporal variables, such as sound intensities of an audio signal, spatial variables, such as luminance values of a picture, or coefficients representing a signal after a suitable transform.
A known method of detecting an embedded watermark in a signal is disclosed in International Patent Application WO-A-99/12347, corresponding to U.S. Pat. No. 6,252,972. The method comprises the steps of correlating the suspect signal with the watermark to be detected, and detecting whether the amount of correlation exceeds a predetermined threshold.
The known detection method is optimal for detecting xe2x80x9cadditively embeddedxe2x80x9d watermarks. The expression xe2x80x9cadditive embeddingxe2x80x9d refers to systems in which the watermark has been added to the original signal in accordance with:
Q=P+sW
where Q is the watermarked signal, P is the original signal, W is the watermark, and s represents the strength of embedding. The same detection method is also used in embedding systems in which the embedding strength is locally adapted to the signal content:
Q=P+S(P)W
where S(P) is referred to as a perceptual mask.
A special class of perceptual masking, which has recently been introduced for image watermarking, is obtained when the embedding strength S(P) is substantially proportional to the signal itself: S(P)=sP. This xe2x80x9cmultiplicative embeddingxe2x80x9d method can be mathematically expressed as
Q=P(1+sW).
It has been found that straightforward correlation of the suspect signal with the watermark is not optimal for detecting the presence of such a multiplicatively embedded watermark.
It is an object of the invention to provide an improved method and arrangement for detecting a watermark, which has been embedded, using a perceptual mask and, more particularly, for detecting a multiplicatively embedded watermark.
To this end, the invention provides a method and arrangement for detecting a watermark in a signal as defined in the independent claims. It has been found by the inventors that the presence of a multiplicatively embedded watermark can be more reliably detected by first squaring the signal samples before correlating the signal with the watermark. More generally, the presence of a watermark, which is embedded, using a perceptual mask should be detected by raising the suspect signal to a power b which is not equal to 1.
Advantageous embodiments of the method and arrangement are defined in the dependent claims.